1. Technical Field
Embodiments of the invention relate generally to integrated circuit structures, and more particularly, but not exclusively, relate to techniques for increasing full well capacity of pixel structures in an image sensor.
2. Background Art
An image sensor converts a visual image into digital data that may be represented as a picture. Digital cameras and video imaging devices employ such image sensors, for example. The image sensor typically includes an array of pixels—unit integrated circuit (IC) structures for the conversion of the visual image into digital data. Image sensors often include complementary metal oxide semiconductor (CMOS) IC elements, for example.
Recently, image sensors have been designed for backside illumination (“BSI”). BSI refers to an arrangement of imaging elements in an image sensor in which one side of a semiconductor substrate, on which photosensitive regions and their associated pixel and image processing circuits are fabricated, is designated as the front side, while the illumination from an object to be imaged is received through the opposite surface of the substrate—or its backside. In BSI image sensors, illumination occurs without interference from any metal or oxide layers that form, for example, the transistor components of the pixel cell and associated interconnects, allowing incident electromagnetic energy a more direct path through the photodiode. In a front-side illumination (“FSI”) image sensor, by contrast, the light from an image strikes the substrate on the side of the substrate where the polysilicon gate electrode, oxide and metal interconnect layers reside, and so care must be taken to ensure that the photo-sensitive region of an FSI pixel cell is not covered by polysilicon or metal layers. Therefore, more electromagnetic energy can typically reach a photodiode in a BSI configuration, which may improve image quality.
Improvements in IC fabrication continue to reduce the size of components in image sensors. Consequently, successive generations of pixel arrays have implemented successively smaller IC design features, such that certain design features are now smaller than the wavelengths of visible light. Such advances have enabled CMOS image sensors, for example, to locate increasingly large numbers of decreasingly small pixels within the same or smaller IC footprints. With decreasing sizes of components in image sensors, it becomes increasingly important to more efficiently capture incident light that illuminates their sensing arrays. That is, more efficiently capturing incident light helps to maintain or improve the quality of electronic images captured by these successively smaller sensing arrays. To the same end, it has become increasingly important to increase, or at least maintain, the photosensitive area within individual pixels—e.g. as compared to the total area allocated to that pixel.